1. Field of the Invention
The present invention relates to a primary side current controller and related power supply, and more particularly, to a primary side current controller and related power supply capable of providing a constant output current.
2. Description of the Prior Art
Power supplies that convert an alternating current (AC) mains voltage to a direct current (DC) voltage for use in integrated electronic devices, among other devices, are known. The power supplies are required to maintain the output voltage, current or power within a regulated range for efficient and safe operation of the electronic device. Switches that operate according to a pulse width modulation (PWM) control to maintain the output voltage, current, or power of the power supply within a regulated range are also known. These switches utilize an oscillator and related circuitry to vary the switching frequency of the switches, and therefore regulate the power, current or voltage that is supplied by the power supply.
In the power supply, it is important to control the output power by regulating the output within a tolerance level and by limiting the maximum output power. Ensuring a maximum power limit helps protect the power supply and connect circuits from damage. However, with increasing of the output voltage, the output current can increase to dangerous levels. It is difficult to control the output current remaining a specific range.
To regulate the output current of the power supply, there have been a number of patents disclosing current controlling methods and apparatuses described as followed.
U.S. Pat. No. 6,977,824 discloses a control circuit for controlling an output current at the primary side of a power converter. The current control circuit comprises a switching control circuit for generating a switching signal for switching a transformer and regulating an output of the power converter.
U.S. Pat. No. 6,972,969 discloses a system and method for controlling current limit with primary side sensing. The system and method control the current limit such that it is maintained within a small range for any acceptable input voltages, and causes the output voltage of a PWM controller to drop as the output load increases when the current limit is reached.
U.S. Pat. No. 7,388,764 discloses a primary side constant output current controller. The primary side constant output current controller comprise a transformer reset time detector, a calculator circuit, an error amplifier, a comparator, a gate drive module, and an oscillator. The transformer reset time detector computes a transformer reset time signal based on a feedback signal. The calculator circuit generates a combined signal by multiplying the transformer reset time signal by a current signal through a power switch. The error amplifier outputs a signal based on the difference between a reference signal and the combined signal. The comparator is configured to compare the combined signal and at least one ramp signal. The gate drive module generates a corresponding gate drive signal operable for properly turning on or off the power switch of the switching regulator. The oscillator generates the oscillator signal to include jitter generated signal digitally as opposed to using an analog technique.